The microdevices manufactured by micro electro mechanical system (MEMS) technology are playing key roles in many areas. For instance, micromechanical gyroscopes have enabled several important control systems in transportation and commercial applications. Other microdevices such as pressure sensors, accelerometers, actuators and resonators fabricated by MEMS technology are also used in many areas.
Some microdevices, such as micro gyroscopes and resonators contain a microstructure that needs to be maintained within a vacuum-sealed cavity. For these types of devices, there is a continuing need to improve the longevity of the vacuum. A vacuum-sealed cavity is typically susceptible to pressure increases due to gas generation during the hermetic sealing process and outgassing from the package material, sealing material, and components within the cavity. This pressure variation can degrade device performance and reduce device lifetime for many hermetically sealed microdevices.
In various applications, it has been known to use certain types of getters to adsorb vapor and gas species. In particular, it has been known to use high sorption getters that are made of porous sintered metallic particles. The use of these types of getters has been met with varying degrees of success. For instance, with the porous metallic getters there is a serious reliability issue caused by getter particles falling down during fabrication process or after the device experiences vibration or shock. It has been found that particles that shed from these getters can jam a sensor element in a microdevice, especially microdevices that have sensor elements that have very fine features on the order of 2 microns. Jamming will cause intermittent or permanent damage to the sensor element.
In other applications, it has been known to use a permeable container that prevents getter particles from contaminating other components such as European Patent Specification, EP 0 720 260 B1, entitled “Getter Housing for Optoelectronic Packages.” This design, however, would add more process steps and increase manufacturing cycle time and cost if applied to microdevice sensor applications.
A non-evaporable getter material is described in U.S. Pat. No. 6,016,034. However, that process needs a separate carrier for the getter material before incorporation into microdevice sensor applications. Again, this design would add more process steps and increase manufacturing cycle time and cost. Moreover, the average grain size of the particles generated form this getter is 10-20 microns or even larger. Any released particles may cause problems if applied to applications using a sensor element with fine features.
For non-metallic getters, these may be formed from organic salts of silicon such as that described in an electron tube application in U.S. Pat. No. 4,771,214. Another non-metallic getter formed from deposited amorphous silicon or poly-silicon for flat panel display applications is described in U.S. Pat. No. 5,614,785. With relation to non-metallic getters, the mechanical properties of known amorphous or poly-crystalline silicon will change with deposition condition and are difficult to repeat. Known types of non-metallic getters are typically used in large sized cavities with large planar areas. Other types of getters are directed to adsorbing moisture within a cavity that is not perfectly hermetic.
The following co-pending patent application describes devices and methods that overcome several of the problems with the application of existing getters in microdevice assemblies: “Hermetically Sealed Microdevices Having a Single Crystalline Silicon Getter for Maintaining Vacuum,” by Xiaoyi Ding, Ser. No. 10/260,675, which is owned by the assignee of the present invention and hereby incorporated by reference herein in its entirety. Although the devices and methods described in that application are helpful, further devices and methods are needed.
It is, therefore, desirable to provide further improved microdevice assemblies having a structure resided in a hermetically sealed cavity to overcome most, if not all, of the preceding problems.
While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims.